Wideband Code Division Multiple Access (WCDMA) has been chosen as the radio technology for the paired bands of the UMTS. Consequently, WCDMA is the common radio technology standard for third-generation wide-area mobile communications. WCDMA has been designed for high-data services and, more particularly, Internet-based packet-data offering up to 2 Mbps in indoor environments and over 384 kbps for wide-area applications.
The WCDMA concept is based on a new general structure for all layers built on technologies such as packet-data channels and service multiplexing. The new concept also includes pilot symbols and a time-slotted structure which has led to the provision of adaptive antenna arrays which direct antenna beans at users to provide maximum range and minimum interference. This is also crucial when implementing wideband technology where limited radio spectrum is available.
The uplink capacity of the proposed WCDMA systems can be enhanced by various techniques including multi-antenna reception and multi-user detection or interference cancellation. Techniques that increase the downlink capacity have not been developed with the same intensity. However, the capacity demand due to the projected data services (e.g. Internet) burdens more heavily the downlink channel. Hence, it is important to find techniques that improve the capacity of the downlink channel.
Bearing in mind the strict complexity requirements of terminals, and the characteristics of the downlink channel, the provision of multiple receive antennas is not a desired solution to the downlink capacity problems Therefore, alternative solutions have been proposed suggesting that multiple antennas or transmit diversity at the base station will increase downlink capacity with minor increase of complexity in terminal implementation.
In third-generation mobile radio systems in general and in particular for WCDMA systems, the downlink capacity is a bottleneck. This is due to fading of the transmitted signal, wherein the amplitude of the signal is subjected to random fluctuations. To overcome this situation, transmitter antenna diversity has been proposed for the downlink direction. Known transmitter diversities schemes can be divided into two categories, open loop systems and closed loop systems. The difference between the open loop and the closed loop systems is that the former sends a feedforward or training information, in order to provide an information about the channel at the receiver. On the other hand, the latter system gets knowledge of the channel at the transmitter side by virtue of a feedback path from the receiver to the transmitter. Selective Transmit Diversity (STD) is an example of a closed loop system which is easy to implement in digital cellular systems due to the presence of a permanent feedback connection. Furthermore, systems that employ either of the two categories of transmitter diversity are known.
The prior art diversity systems are described e.g. in document U.S. Pat. No. 5,832,044 and in the publications “Fading Resistant Modulation Using Several Transmitter Antennas” by Sousa et al., IEEE Trans. On Communications, pp. 1236-1244, October 1997, and “Diversity Transform for Fading Channels”, by D. Rainish, IEEE Trans. On Communications, pp. 1653-1661, December 1996.
In the above prior art systems, all components of a constellation vector (super symbol) are transmitted via either of different antennas, different carrier frequencies, or diferent time slots. However, since the optimum decoding complexity grows exponentially with the number of components of the constellation vector, the transmission capacity is limited. Moreover, a high peak to average ratio results from an increased constellation size.